Little-studied metal goes critical. (Neptunium Nukes?).

Scientists first created the artificial element neptunium in 1940, around the time they made the first atoms of plutonium. But because plutonium turned out to be so much better for nuclear weapons, researchers have paid scant attention to neptunium.

Now a team at Los Alamos (N.M.) National Laboratory has measured how much neptunium it would take to make a bomb--the element's critical mass--with far greater accuracy than ever before. From a preliminary data analysis, the investigators find that neptunium's critical mass is around 60 kilograms. The critical mass of plutonium-239 is about 10 kg, and for uranium-235, the most widely used nuclear explosive after plutonium, it's about 50 kg.

The new measurement of neptunium-237, the artificial metal's most common and stable isotope, shows that it "is about as good a bomb material as U-235," comments Richard L. Garwin of IBM's Thomas J. Watson Research Center in Yorktown Heights, N.Y.

Commercial nuclear reactors create annually tons of neptunium mixed in with other nuclear waste. Indeed, since 1999, the International Atomic Energy Agency in Vienna has urged monitoring of neptunium. However, according to the agency, the risk of anyone making neptunium bombs from that highly radioactive waste is low because it would be so difficult to extract the isotope.

Since its inception more than a half century ago, the U.S. nuclear-weapons program has generated about 340 kg of pure neptunium, says Rene G. Sanchez, who co-led the critical-mass measurement with David J. Loaiza.

"All the [pure] neptunium in this country is tracked and accounted for," says Los Alamos team member Steven D. Clement. "The question mark is other countries." Russia probably holds significant quantities of pure neptunium, he adds.

When struck by neutrons, nuclei of neptunium and other nuclear materials can fission into two smaller nuclei. Because fission releases additional neutrons, a chain reaction can take place. A critical mass produces a self-sustaining chain reaction. If controlled in a nuclear reactor, such a chain reaction can safely generate power. If uncontrolled, the chain reaction can unleash a mighty blast.

To measure neptunium-237's critical mass, the Los Alamos team cast a softball-size, 6-kg sphere of the metal that could nestle within two hemispheres of uranium. The researchers then brought the uranium hemispheres together over the course of 4 days at a carefully controlled rate.

When the hemispheres were about 3 millimeters apart, detectors recorded a steep rise in neutron production, signaling that a chain reaction was, in nuke parlance, going critical. From the neptunium-uranium assembly's critical mass, the team then calculated neptunium's individual critical mass. "When you go critical, that's when things get exciting," Clement says.

That's exactly the kind of excitement that designers of nuclear-waste repositories want to avoid. "If you're going to store it, you have to know how much you can put together and still be safe," says nuclear-safety specialist Robert D. Busch of the University of New Mexico in Albuquerque.